A conventional design for a midframe design of a can-annular gas turbine engine 110 is illustrated in FIG. 1. A compressor 111 directs compressed air through an axial diffuser 113 and into a plenum 117, after which the compressed air turns and enters a sleeve 122 positioned around a combustor 112. The compressed air is mixed with fuel from various fuel stages 119 of the combustor 112 and the air-fuel mixture is ignited at a stage 121 of the combustor 112. Hot combustion gas is generated as a result of the ignition of the air-fuel mixture, and the hot combustion gas is passed through the combustor 112 and into a transition 114, which directs the hot combustion gas at an angle into a turbine 115.
In conventional can-annular gas turbine engines, a lean air/fuel mixture is ignited at the stage 121 of the combustor 112. However, at high loads and high temperatures, various emissions, such as nitrous oxide (NOx), are generated within the hot combustion gas as a result of igniting the lean air/fuel mixtures, and these emissions may exceed legally permissible limits. Additionally, if a rich air/fuel mixture is ignited at the stage 121 of the combustor 112, the temperature of the generated combustion gas may not be sufficient to combust hydrocarbons present within the combustion gas and thus the hydrocarbons may also exceed legally permissible limits.
In addition to the conventional design discussed above, U.S. Pat. No. 6,192,688 to Beebe discloses a combustion stage arrangement in a gas turbine engine, in which a lean air-fuel mixture is injected into combustion gas at a downstream stage from an upstream stage where a lean air-fuel premixture is combusted to generate the combustion gas. Additionally, other combustion stage designs have also been proposed in U.S. Pat. No. 5,271,729 to Gensler et al. and U.S. Pat. No. 5,020,479 to Suesada et al. However, these designs are for non-gas turbine combustion arrangements.
In the present invention, the present inventors make various improvements to the combustion stage design of the can-annular gas turbine engine, to overcome the noted disadvantages of the conventional combustion stage design.